The invention relates to polyethylene blends. The blends comprise a high molecular weight, medium density polyethylene (HMW MDPE) and a linear low density polyethylene (LLDPE). The invention also relates to films made from the blends.
Polyethylene is divided into high density (HDPE, density 0.941 g/cc or greater), medium density (MDPE, density from 0.926 to 0.940 g/cc), low density (LDPE, density from 0.910 to 0.925 g/cc) and linear low density polyethylene (LLDPE, density from 0.910 to 0.925 g/cc). (See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials.) One of the main uses of polyethylene (HDPE, LLDPE, and LDPE) is in film applications, such as grocery sacks, institutional and consumer can liners, merchandise bags, multi-wall bag liners, produce bags, deli wrap, and shrink wrap. The key physical parameters of polyethylene film include tear strength, impact strength, tensile strength, stiffness and clarity. Tear strength is measured in machine direction (MD) and transverse direction (TD). Total tear strength (the product of MD tear and TD tear) is an indicator of overall tear properties. Critical processing properties on the film line include the output, bubble stability, gauge control (variability in film thickness), extruder pressure and temperature.
Film stiffness can be measured by modulus. Modulus is the resistance of the film to deformation under stress. It relates to its density. A higher density gives a higher modulus. A typical LLDPE film has a modulus of about 32,000 psi, while a HDPE film has a modulus of about 100,000 psi or higher. LLDPE film has higher impact strength and MD tear, while HDPE has higher stiffness and tensile strength. When LLDPE producers attempt to increase the density (thereby increasing the modulus of the film), they often encounter losses in impact strength and MD tear. Historically, blending LLDPE and HDPE has not achieved xe2x80x9cbreakthroughxe2x80x9d success. The blends often give films that have improved stiffness and tensile properties, but the impact and tear properties are usually sacrificed. There are no straightforward methods or single resins that have the combined properties of both.
While there are few polyethylene films of modulus between about 40,000 psi and about 90,000 psi, there is an increasing demand for such films. For example, gardening has become one of the largest hobbies by dollars spent in the United States. To support gardeners, a variety of products need to be commercially available in large and small packages. Because consumer perception is important, the bags need to possess a high aesthetic appearance and excellent mechanical integrity. When consumers load 50-lb bags of fertilizer or pesticide into their cars, they need to feel comfortable and safe. This requires the bags to be easy to handle and stack, to resist puncture and tear propagation, to have good sealability and excellent seal strength, and to be glossy and printable. Existing films for these markets are primarily LLDPE resins. Although HDPE films are more similar to the paper packaging that they have replaced in these industries, HDPE films do not have the impact and tear properties essential for acceptable durability standards.
Recently, a high molecular weight, medium density polyethylene (HMW MDPE) has been developed (see copending appl. Ser. No. 09/648,303 (docket No. 88-1026A) filed on Aug. 25, 2000). The HMW MDPE has many unique properties and offers new opportunities for improvement of polyethylene films.
The invention is a blend comprising a high molecular weight, medium density polyethylene (HMW MDPE) and a linear low density polyethylene (LLDPE). The blend comprises from about 20 wt % to about 80 wt % of HMW MDPE. The HMW MDPE has a density from about 0.92 to about 0.944 g/cc, a melt index MI2 from about 0.01 to about 0.5 dg/min, and a melt flow ratio MFR from about 50 to about 300. It has a multimodal molecular weight distribution comprising a high molecular weight component and a low molecular weight component. The low molecular weight component has an MI2 from about 50 to about 600 dg/min and a density from about 0.94 to about 0.97 g/cc. The blend also comprises about 20 wt % to about 80 wt % of LLDPE. The LLDPE has a density within the range of about 0.90 to about 0.925 g/cc and an MI2 within the range of about 0.50 to about 50 dg/min.
The invention also includes a film prepared from the blend and a method for making the film. We have surprisingly found that blending the HMW MDPE and a high performance, conventional or single-site LLDPE gives the film toughness and tear strength similar to LLDPE with stiffness and tensile properties similar to medium density HDPE films.
The blend of the invention comprises from about 20 wt % to about 80 wt % of a high molecular weight, medium density polyethylene (HMW MDPE). Preferably, the blend comprises from about 30 wt % to about 70 wt % of HMW MDPE. The HMW MDPE has a density within the range of about 0.92 to about 0.944 g/cc. Preferably, the density is within the range of about 0.935 to about 0.944 g/cc. Preferred HMW MDPE is a copolymer that comprises from about 85 to about 98 wt % of recurring units of ethylene and from about 2 to about 15 wt % of recurring units of a C3 to C10 xcex1-olefin. Suitable C3 to C10 xcex1-olefins include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, and 1-octene, and the like, and mixtures thereof.
The HMW MDPE has an MI2 from about 0.01 to about 0.5 dg/min, preferably from about 0.01 to about 0.3 dg/min, and an MFR from about 50 to about 300. Melt index (MI2) is usually used to measure polymer molecular weight, and melt flow ratio (MFR) is used to measure the molecular weight distribution. A larger MI2 indicates a lower molecular weight. A larger MFR indicates a broader molecular weight distribution. MFR is the ratio of the high-load melt index (HLMI) to MI2. The MI2 and HLMI can be measured according to ASTM D-1238. The MI2 is measured at 190xc2x0 C. under 2.16 kg pressure. The HLMI is measured at 190xc2x0 C. under 21.6 kg pressure. The HMW MDPE has a considerably higher molecular weight (or lower MI2) and broader molecular weight distribution (or larger MFR) than conventional HDPE or LLDPE.
The HMW MDPE has a multimodal molecular weight distribution. By xe2x80x9cmultimodal molecular weight distribution,xe2x80x9d we mean not only that the HMW MDPE has at least two different molecular weight components, but also that the two components differ chemically and structurally from each other. The low molecular weight component has an MI2 within the range of about 50 to about 600 dg/min, while the high molecular weight component preferably has an MI2 less than about 0.5 dg/min. The high molecular weight (low MI2) component gives the polyethylene superior bubble stability in a blown film process and the low molecular weight (high MI2) component gives the polyethylene excellent processability. Furthermore, the low molecular weight component has a density from about 0.94 to about 0.97 g/cc (i.e., in the range of conventional HDPE), while the high molecular weight component preferably has a density from 0.90 to 0.94 g/cc, more preferably from 0.91 to 0.94 g/cc, which is similar to the conventional LLDPE.
Copending appl. Ser. No. 09/648,303 (docket No. 88-1026A) filed on Aug. 25, 2000, the teachings of which are herein incorporated by reference, teaches preparation of the HMW MDPE by a multiple zone process with Ziegler catalysts. For example, a HMW MDPE can be produced by polymerizing an olefin mixture containing from about 85 to about 98 wt % of ethylene and from about 2 to about 15 wt % of a C3 to C10 xcex1-olefin in a first reaction zone to produce a first polymer, removing some volatile materials from the first polymer, and then continuing the polymerization in a second reaction zone by adding more of the olefin mixture.
The blend of the invention comprises from about 20 wt % to about 80 wt % of a linear low density polyethylene (LLDPE). Preferably, the blend comprises from about 30 wt % to about 70 wt % of LLDPE. The LLDPE has a density within the range of about 0.90 to about 0.925 g/cc and an MI2 within the range of about 0.5 to about 50 dg/min. LLDPE can be produced by Ziegler catalysts or newly developed single-site catalysts. Ziegler catalysts are well known. Examples of suitable Ziegler catalysts for making LLDPE include titanium halides, titanium alkoxides, vanadium halides, and mixtures thereof. Ziegler catalysts are used with cocatalysts such as alkyl aluminum compounds.
Single-site catalysts can be divided into metallocene and non-metallocene. Metallocene single-site catalysts are transition metal compounds that contain cyclopentadienyl (Cp) or Cp derivative ligands. For example, U.S. Pat. No. 4,542,199, the teachings of which are incorporated herein by reference, teaches metallocene catalysts. Non-metallocene single-site catalysts contain ligands other than Cp but have the same catalytic characteristics as metallocenes. The non-metallocene single-site catalysts may contain heteroatomic ligands, e.g., boraaryl, pyrrolyl, azaborolinyl or quinolinyl. For example, U.S. Pat. Nos. 6,034,027, 5,539,124, 5,756,611 and 5,637,660, the teachings of which are incorporated herein by reference, teach non-metallocene catalysts.
LLDPE resins are usually copolymers of ethylene with 5 to 15 wt % of a long chain xcex1-olefin such as 1-butene, 1-hexene, and 1-octene. Higher puncture resistance and tear strength are typical of LLDPE films. Great impact resistance and tear strength can be achieved by using 1-octene as the comonomer. Conventional 1-hexene based LLDPE is inferior to that made with 1-octene. However, higher performance 1-hexene based LLDPE, which has comparable properties to 1-octene based LLDPE, has been developed (see, e.g., U.S. patent appl. Ser. No. 09/205,481, filed Dec. 4, 1998). Usually, when conventional HDPE and LLDPE are blended, the blend does not perform synergistically. However, we have surprisingly found that when LLDPE is blended with the newly developed HMW MDPE described above, the blend exhibits better performance than the sum of the single components. We believe that these benefits result from the improved compatibility of the more amorphous HMW-MDPE and much broader MWD compared to conventional MDPE or HDPE resins. Also, by blending HMW-MDPE with LLDPE, film densities can be achieved as low as current commercial LLDPE offerings and as high as commercial MDPE offerings, thus bridging the modulus gap between LLDPE and HDPE without sacrificing impact and tear properties.
Optionally, the blend contains a third polymer. Adding a third polymer into the blend can either enhance the performance of the product or reduce the cost. For example, addition of a third polymer may increase the printability or the clarity of the film. Suitable third polymers include polyethylene resins other tan specified above, e.g., low density polyethylene (LDPE) and HDPE, polypropylene, polyester, acrylic resin, polyvinyl alcohol, polyvinyl chloride, polyvinyl acetate, polyvinyl ether, ethylene-vinyl acetate copolymers (EVA), ethylene-vinyl alcohol copolymers (EVOH), ethylene-acrylic acid copolymers, and the like, and mixtures thereof. A third polymer is added in an amount preferably less than 50 wt % of the total blend.
Optionally, the blend also contains antioxidants, UV-absorbents, flow agents, or other additives. The additives are well known in the art. For example, U.S. Pat. Nos. 4,086,204, 4,331,586 and 4,812,500, the teachings of which are herein incorporated by reference, teach UV stabilizers for polyolefins. Additives are added in an amount preferably less than 10 wt % of the total blend.
Any suitable blending technique can be used. The polymers and optional additives can be blended in solution or in thermal processing. Melt screw extrusion is preferred. The resulting blend preferably has a density within the range of about 0.925 to about 0.935 g/cc and an MI2 within the range of about 0.1 to about 0.5 dg/min.
The invention includes films made from the blends. The films include films of thickness less than 10 mils and sheets of thickness greater than 10 mils. One advantage of the invention is that the blend film can be produced with conventional film equipment for LLDPE or on high stalk film equipment for HMW-HDPE. For typical HDPE or MDPE resins, lowering the density may reduce the bubble stability while processing on a high stalk film line. In spite of the lower density, the blend film of the invention exhibits excellent bubble stability on a high stalk extrusion line. The blend film can be produced on either a high stalk film line or a conventional, in-the-pocket LLDPE film line. Another advantage of the blend film is that it exhibits superior total tear properties compared to conventional HDPE or MDPE. With the lower density, the blend film feels softer than the conventional is HDPE or MDPE films. Yet the films have much better tensile strength than that produced from LLDPE resins, resulting in good handle and yield strength in bags. In thicker film gauges used in heavy duty shipping sacks, the blend film exhibits outstanding tear and impact properties.
Methods for making polyethylene films are known. For example, U.S. Pat. No. 5,962,598, the teachings of which are herein incorporated by reference, teaches how to produce biaxially oriented films made in high stalk extrusion. In the process, polyethylene melt is fed by an extruder through a die gap (0.8 to 2 mm) in an annular die to produce a molten tube that is pushed vertically upward. At this point, the molten tube is approximately the same size as the annular die. Pressurized air is fed to the interior of the tube to increase the tube diameter to give a xe2x80x9cbubble.xe2x80x9d The volume of air injected into the tube controls the size of the tube or the resulting blow-up ratio. In high stalk extrusion, the increase in the tube diameter occurs at a height of approximately 5-12 times the die diameter. This distance is referred to as the stalk or neck height. The expanded tube produces the desired biaxial orientation of the film that results in the balance of tear and impact properties of HMW HDPE resins. The tube is rapidly cooled by a cooling ring on the outside surface of the film. The bubble is collapsed between a pair of nip rollers and wound onto a film roll by the film winder. Collapsing of the tube is done after initial cooling at a point so that the wall surfaces will not adhere to one another. Mechanical strength of the film is defined in two directions, along the polymer flow exiting the die or machine direction (MD) and perpendicular to the polymer flow exiting the die or transverse direction (TD).
The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.